Electrically variable focus polymer-stabilized liquid crystal lens having non-homogenous polymerization of a nematic liquid crystal/monomer mixture

ABSTRACT

A variable focus liquid crystal lens includes a nematic liquid crystal/monomer mixture having a spatially inhomogenous polymer network structure, and an electrode for applying a substantially uniform voltage to the nematic liquid crystal/monomer mixture. The lens is created within a cell by applying a substantially uniform electric field to the nematic liquid crystal/monomer mixture within the cell, while simultaneously irradiating the nematic liquid crystal/monomer mixture using a laser beam having a shaped intensity distribution, so as to induce formation of a spatially inhomogenous polymer network structure within the cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/535,089, filed Sep. 26, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/861,480 (now U.S. Pat. No. 7,218,375) filed Jun.7, 2004, which claims priority to U.S. provisional patent applicationNo. 60/475,900 filed Jun. 5, 2003.

TECHNICAL FIELD

The present invention relates generally to electro-optical devices, andin particular to an electrically variable focus polymer-stabilizedliquid crystal lens.

BACKGROUND OF THE INVENTION

Different ways of designing variable focal length lenses based on flatlayers of nematic liquid crystals (NLC) are known in the art. Typically,a non-homogeneous electric field is used to induce a suitableconfiguration of the NLC director (that is, the direction of thepreferred molecular orientation) in a cell so as to create a lens-likedistribution of the refractive index. Non-homogeneous electric fieldscan be generated by means of suitable electrode structures provided onone or both cell substrates.

In some cases, a small amount (e.g., up to 3%) of a reactive monomer isadded to the NLC. The reactive monomer is substantially uniformlypolymerized in situ by uniform UV irradiation during application of thenon-homogenous electric field. Polymerization of the monomer in thismanner leads to the formation of a spatially uniform polymer networkstructure or matrix, which reduces the ease with which the NLC directorcan be reoriented. Accordingly, polymerization of the monomer while theNLC while under the influence of the non-uniform electric field reducesthe tendency of the NLC director to re-orient back to its relaxed statewhen the electric field is removed, thereby producing a “permanent” lenswithin the NLC. The accuracy of control of the focal length of such alens depends on the concentration of monomer. Another known method offorming micro-lenses, of fixed focal length, is to use strongly focusedlight to induce LC reorientation and simultaneous UVphotopolymerization.

Different polymer network structures and their influence on theelectrical switching properties of NLC have been studied. In particular,for the structure of nematic domains separated by thin polymeric walls,it has been established that the threshold field of nematicreorientation increases as the density of the polymer network increases.Recently, patterned irradiation has been used to produce regions withdifferent threshold voltage for switching. When used in conjunction witha mask in the form of concentric dark and transparent rings, thistechnique can be used to produce a switchable Fresnel lens.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electricallyvariable focus polymer-stabilized liquid crystal lens, and methods formaking same.

Accordingly, an aspect provides a variable focus liquid crystal lenscomprising: a cell containing a nematic liquid crystal/monomer mixturehaving a spatially non-homogenous polymer network structure distributednon-uniformly within the nematic liquid crystal monomer mixture; and atleast one electrode for applying a substantially uniform voltage to thenematic liquid crystal/monomer mixture in the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a cross-sectional view schematically illustrating a cellusable for generating a lens in accordance with an embodiment of thepresent invention;

FIG. 2 is a perspective view of the cell of FIG. 1;

FIG. 3 is a block diagram schematically illustrating principal elementof an experimental apparatus for generating and analyzing a lens inaccordance with the present invention;

FIG. 4 is a graph showing representative variations in lighttransmittance through two points of the cell of FIG. 1 with changingapplied voltage;

FIGS. 5 a-5 c are graphs showing representative variations in lighttransmittance across the cell of FIG. 1, and different applied voltages;

FIG. 6 is a graph showing representative variations in phase differencewith changing applied voltage;

FIG. 7 is a graph showing representative variations in lens focal lengthwith changing applied voltage.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an electrically variable focal lengthlens, and methods of making same. FIGS. 1 and 2 illustrate the structureof a lens in accordance with the present invention. An experimentaldemonstration of a process of making the lens, and properties of a lensproduced by this method are then described with reference to FIGS. 3-7.

Stated very broadly, a variable focal length lens in accordance with thepresent invention is generated by inducing the formation of a spatiallynon-homogenous polymer network within an NLC/monomer mixture containedwithin a cell, in the presence of a uniform electric field.

As shown in FIGS. 1 and 2, the cell 2 can conveniently be defined by apair of substantially parallel transparent substrates 4 separated by agap that is filled with the NLC/monomer mixture 6. Each substrate 4(which may, for example, be made of glass) includes a transparentelectrode 8 (e.g. of Tin Oxide; Indium Tin Oxide (ITO) etc.) coated witha surfactant (e.g. rubbed PMMA) to define a uniform rest-stateorientation of the NLC director. The electrodes 8 are connected to avoltage source 10, which enables the generation of a substantiallyuniform electric field through the NLC/monomer mixture 6 within the cell2. The gap between the substrates 4 may have any suitable thickness(e.g. about 4 μm).

In preferred embodiments, the spatially non-homogenous polymer network12 (FIG. 2) is centro-symmetric, which can be formed by irradiation witha laser beam 14 having a Gaussian energy distribution across the beam,as may be seen in FIG. 1.

A uniform electric field is used during formation of the polymer network12 in order to define a substantially uniform orientation of the NLCduring this process. In some embodiments, the uniform electric field isof zero strength, in which case the uniform orientation of the NLCduring polymerization corresponds with the rest-state orientation of theNLC director defined by the surfactant. Alternatively, a uniformnon-zero electric field may be used to define a desired NLC directororientation during polymerization.

The NLC/monomer mixture 6 may be composed of any suitable nematic liquidcrystal and a small amount (e.g., about 3% by weight) ofphotopolymerizable monomer. The NLC may be a commercially available NLC,such as E7 (by Merck). The photopolymerizable monomer may be amonofunctional monomer (such as glycidyl methacrylate) which contains anepoxy group (e.g., SR-379, from Sartomer Company) and a photoinitiationcomplex (i.e., dye, initiator and coinitator) known in the art.

The laser beam 14 can be generated by any suitable laser source (notshown in FIGS. 1 and 2), and has a frequency selected to correspond tothe range of sensitivity of the monomer. For example, a Verdi laser beam(λ=532 μm) having a desired diameter (e.g., about 2.5 mm) can be used inconjunction with the above-noted NLC/monomer mixture 6. Suitable opticalelements (lenses, mirrors etc) can be used in a manner known in the artto collimate the laser beam 14 at the desired diameter, define aGaussian energy distribution across the beam, and direct the beamsubstantially normally through the cell 2.

With this arrangement, irradiation of the NLC/monomer mixture 6 by thelaser beam 14 causes the monomer lying within the irradiated region topolymerize, thereby forming a 3-dimensional polymer network structure 12(FIG. 2) within the irradiated region. The density of the networkstructure is proportional to the duration of the irradiation, and theoptical power. Consequently, these parameters can be selected to inducea non-homogenous polymerization density, which follows thecentro-symmetric Gaussian energy distribution of the laser beam 14.

As is known in the art, polymerization of the monomer increases thethreshold electric field required to induce re-orientation of NCL, andthe magnitude of the threshold increase is proportional to the densityof the polymer network. Thus, non-homogenous polymerization of themonomer causes the NLC within the irradiated/polymerized region toexhibit a corresponding non-homogeneous electro-optical response to auniform electric field.

More particularly the threshold field strength (voltage) for directorreorientation will be maximal in the center of the polymerized region12, and minimal in the surrounding non-illuminated region 16. As aresult, the application of a uniform electric field to the cell 2produces a corresponding distribution of the nematic director, to form acircularly symmetric distribution of the refractive index {right arrowover (n)}, with a maximum in the center of the polymerized region 12.Such a cell 2 will represent a LC lens. Changes in the applied voltagewill vary the profile of the refractive index, and the focal length ofthe lens. An experimental demonstration of the process, and propertiesof the resulting lens will now be described with reference to FIGS. 3-7.

FIG. 3 schematically illustrates an experimental apparatus used togenerate and examine a variable focal length lens in accordance with thepresent invention. As shown in FIG. 3, the apparatus includes an LC cell2 as described above with reference to FIGS. 1 and 2; a Verdi laser 18for polymerizing the monomer to generate the lens within the cell 2, asdescribed above. A He—Ne laser 20 generates a narrow-beam probe that isdetected by a photodiode 22 for analyzing properties of the lens.

Photopolymerization of the monomer was induced by means of the Verdilaser beam (λ=532 μm) having Gaussian intensity distribution asdescribed above. In this particular trial, the diameter of the beam was2.3 mm. The cell was irradiated for 30 minutes at a total power of 23.8mW. No electric field was applied to the NLC during irradiation.

The He—Ne laser beam (λ=543.5 μm, diameter 0.7 mm) was used as a probeat normal incidence on the cell 2. In order to analyze the properties ofthe lens, the intensity (I) of He—Ne light transmitted through the cell2 is analyzed as a function of position (X) across the irradiated(polymerized) region 12. The rubbing direction of the PMMA surfactant inthe cell 2 corresponded to axis X and was oriented at 45° with respectto crossed Glan prism 24 (used as a polarizer) and an analyzer. Theintensity (I_(max)) of light transmitted through the parallel polarizerswas measured also to take into account the absorption and reflectionlosses on polarizers. The cell 2 was mounted on a movable stage 26,allowing examination of different points of the cell 2 in the Xdirection. The intensity of the probe beam was attenuated by means of aneutral filter 28 to minimize its influence on the NLC/monomer mixturewithin the cell. The light transmitted through the cell 2 was detectedby the photodiode 22. The electric field within the cell was generatedby a signal generator 30 connected to apply a sinusoidal signal with 1kHz frequency to the electrodes 4. The r.m.s. value of the appliedvoltage was monitored using a numerical multimeter.

As may be seen in FIG. 4, the dependences of the normalized lighttransmission T=I/I_(max) on the applied voltage measured at the centralpoint of polymerized region 12 (solid line) and at the non-polymerizedregion 16 (dashed line) are equivalent qualitatively, but are shiftedwith respect each other along the voltage axis. This fact indicates thesimilar character of the field-induced nematic reorientation at thesepoints. However the reorientation at the polymerized point requireshigher voltage, than the same reorientation at the non-polymerized one.It should be note that an oscillated behavior of dependence of T on theexternal field is also typical for the case of a pure planar nematiclayer.

As shown in FIG. 5 a, the light transmission before polymerization as afunction of the probe beam position X in the cell 2 is shown fordifferent values of applied voltages. The interval of X from 2 mm to 7.5mm corresponds to the region of the cell, which is filled with theNLC/monomer mixture. Within this portion of the cell T≠0 due to thebirefringence of the NLC. Outside of this region the empty cell isoptically isotropic and the T=0. As can be seen from FIG. 5 a, T isgenerally constant everywhere in the filled region at U=0. This opticalhomogeneity indicates that the orientation of the NLC in the cell isuniform. Under the influence of the electric field the lighttransmission varies. The curves represented in FIG. 5 a for U=1.01 V and1.06 V show that T is constant in the filled area, except the smallregions near the borders (at λ=2.3 and 7 mm), where the edge effectsbecome apparent.

FIG. 5 b shows the same variation in light transmission with appliedvoltage, but measured after the process of photopolymerization. As maybe seen in FIG. 5 b, the optical homogeneity of the cell is more or lesspreserved for U=0. However an applied voltage produces non-homogeneouschanges of the light transmission. Thus at U>0 the light transmissioncurves have clearly expressed peaks with a maximum corresponding to thecenter of the polymerizing beam 14. It should be noted that the voltagevalues 1.01V and 1.06V in FIG. 5 b do not correspond to the situationpresented in FIG. 4, because the measurements were made for twodifferent cells having different thickness. In the inset of FIG. 5 b theGaussian intensity distribution of the polymerizing beam 14 is shown.The center of the beam coincides with the position X_(c), and the formof the light transmission peaks approximately reproduces the Gaussianenergy profile of the polymerizing beam 14. The peak amplitude decreaseswith increasing applied voltage, and disappears completely for highvoltage values. In FIG. 5 c the results for the light transmissionbefore and after photopolymerization at high voltage are presented. Thecurves coincide at every point of the cell that indicates anuniformization of the cell (homeotropic alignment of NLC).

The centro-symmetric character of the electro-optical response of thecell shown in FIG. 5 b indicates a similar distribution of the effectiverefractive index in the polymerized area 12. Such distribution is due tothe centro-symmetric character of nematic reorientation, which dependsstrongly on the structure of the polymer network induced by thepolymerizing laser beam 14. Since the polymerization rate isproportional to the intensity of light, polymerization appears to startfrom the center of the irradiated spot, and propagate in the plane ofthe substrates 4, with circular symmetry. As a consequence, and due tothe diffusion of monomer to the brighter regions, the density of thepolymer network is maximal in the center of irradiated spot anddecreases toward the outer regions. Higher voltage is necessary toreorient the nematic confined in polymer network with higherconcentration. Thus the retardation of the nematic reorientation isobserved in the center of polymerized area 12 with respect to the edges.At much higher values of applied voltage, the influence of the polymernetwork become negligible compared to the electric field, and thenematic reorients uniformly substantially throughout the cell (see FIG.5 c).

FIG. 6 shows the dependence of the phase difference δF=φ(X_(c))−φ(X_(b))on the applied voltage in which, φ being the induced phase difference ofordinary and extraordinary waves at the given point; X_(c) and X_(b) arethe coordinates of the center and the border of the photopolymerizedspot, respectively (see FIG. 5 b). φ has been calculated from therelation:

$I = {I_{\max}{{Sin}^{2}\left( \frac{\varphi}{2} \right)}}$

The maximum difference δF is achieved at the voltages lightly above thethreshold value. This difference decreases with increasing voltage andeventually becomes zero for high voltages.

As shown in FIG. 7, the effective focal length varies inversely with δFin the range of voltages immediately above the threshold voltage U_(th)(=0.98 V in FIG. 7). Within this range, the focal length may becalculated using the expression

${f = \frac{\pi\; a^{2}}{{\lambda\delta}\; F}},$where a=X_(c)−X_(b) is the radius of the lens.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1. A method of making a variable focus liquid crystal lens, the methodcomprising inducing formation of a spatially non-homogenous densitypolymer network structure distributed within a volume of liquid crystalof a liquid crystal cell of said lens, the polymer network structurehaving a predetermined spatial density variance that creates acorresponding spatial variance in the electro-optical response of theliquid crystal.
 2. A method as claimed in claim 1, wherein said liquidcrystal cell is filled with a mixture of a liquid crystal material and aphotopolymerizable monomer, and wherein said inducing comprisesirradiating the liquid crystal/monomer mixture with light having anon-homogenous intensity distribution so as to induce formation of thespatially non-homogenous polymer network structure within the cell.
 3. Amethod as claimed in claim 2, wherein said polymer network structure isless than 3% by weight of a combined weight of said polymer networkstructure and said liquid crystal material.
 4. A method as claimed inclaim 3, wherein said monomer is about 3% of said mixture by weight. 5.A method as claimed in claim 2, wherein said irradiating comprisesirradiating using a laser light source.
 6. A method as claimed in claim5, wherein the intensity distribution is circularly symmetrical.
 7. Amethod as claimed in claim 6, wherein the intensity distribution is aGaussian distribution.
 8. A method as claimed in claim 6, wherein saidinducing is carried out while subjecting the liquid crystal/monomermixture within said cell to a uniform electric field.
 9. A method asclaimed in claim 2, wherein said inducing is carried out whilesubjecting the liquid crystal/monomer mixture within said cell to auniform electric field.
 10. A method as claimed in claim 9, furthercomprising a step of providing a surfactant for orientating the liquidcrystal/monomer mixture.
 11. A method as claimed in claim 2, wherein theintensity distribution is circularly symmetrical.
 12. A method asclaimed in claim 11, wherein the intensity distribution is a Gaussiandistribution.
 13. A method as claimed in claim 2, wherein the liquidcrystal material is nematic.
 14. A method as claimed in claim 1, whereinthe liquid crystal is nematic.
 15. A method as claimed in claim 1,wherein said inducing is carried out while subjecting a liquid crystalwithin said cell to a uniform electric field.
 16. A method as claimed inclaim 15, further comprising a step of providing a surfactant fororientating the liquid crystal/monomer mixture.
 17. A method as claimedin claim 1, wherein said liquid crystal cell contains aphotopolymerizable monomer, and wherein said inducing comprisesirradiating the cell using light to induce formation of a spatiallynon-homogenous density polymer network structure within the cell.
 18. Amethod of making a variable focus liquid crystal lens, the methodcomprising irradiating a liquid crystal/monomer mixture within a cellwith light having a non-homogenous intensity distribution so as toinduce formation of a spatially non-homogenous polymer network structurewithin the cell, the polymer network structure having a predeterminedspatial density variance that creates a corresponding spatial variancein the electro-optical response of the liquid crystal.
 19. A method asclaimed in claim 18, further comprising applying a substantially uniformelectric field to the liquid crystal/monomer mixture within the cellsimultaneously with the irradiating.
 20. A method as claimed in claim18, further comprising providing a surfactant for orientating the liquidcrystal/monomer mixture.
 21. A method as claimed in claim 18, whereinsaid irradiating comprises using a laser light source to generate thelight.